Method of forming bowl shaped screen for electron discharge tubes



1964 M. F. TOOHIG ETAL METHOD OF FORMING BOWL SHAPED SCREEN FOR ELECTRON DISCHARGE TUBES Filed May 17. 1960 a wm 4 1 z 6 i m. \\\a H m /H m \w IMQM VQ M 4% F mw m 9 n i fi g/m fi h a M m M W M M N am a? E m z w i m mg m H M 5 M fi m 3 m w m z g N a F M 1 Wm M w m M r M a W w 6 e a 5 m u e w M a a w s m W C Y M. WW mm m mm Wm; mm M m 7M 5 aw M w W 2% W F F W m WW Z}: 2

United States Patent 3,152,323 NETHGD 8F FGRtt ih JG SHAPED SCREEN F913 ELECTRGN BlSCI-IARGE liJhES Michael F. Toohig and Albert 5. Knight, Fort Wayne,

Ind, assignors to international Telephone and Telegraph Corporation Filed May 17, 1969, st. No. 29,649 Claims. (61. -2513) This invention relates generally to methods of forming bowl-shaped screens for electron discharge tubes, and more particularly to methods of forming bowl-shaped target electrode assemblies for barrier grid storage tubes.

Barrier grid storage tubes are well known in the art, being shown for example in Patent No. 2,538,836 of January 23, 1951, to A. S. Jensen. Such tubes, which are of the signal-to-signal type, conventionally include an electron gun assembly including a cathode heated by a suitable filament, a control grid, and an accelerating anode, all positioned within an elongated envelope at one end thereof. Suitable deflecting and focusing elements are conventionflly provided for causing the electron beam produced by the electron gun to scan a target electroce assembly positioned within the envelope at the other end thereof.

The target electrode assembly comprises a fine mesh metal grid or screen arranged on one side of a sheet of dielectric material with a metal backing plate arranged on the other side thereof. The electron beam from the electron gun is caused to scan the target electrode assembly thereby causing emission of secondary electrons from the dielectric sheet. Each square of the target electrode formed by the screen in essence forms 21 separate capacitor with a metal backing plate, and thus may be charged by the electron beam. These charges may subsequently be taken off the target electrode assembly by subsequent scanning by the beam.

In the past, barrier grid storage tubes have commonly employed a flat target electrode assembly with a sheet of mica serving as the dielectric layer and with the screen mechanically attached thereto. Such tubes were subject to microphonics due to vibration of the screen caused by external jarring of the tube and also caused by rapid reversal of the charge on the target electrode. In order to eliminate the problem of microphonics, it has been proposed to employ, as the dielectric layer, a layer of ceramic material, such as glass, fuzed to the metal backing plate with the fine mesh metal screen in turn being fuzed thereto.

Developments in the barrier grid storage tube field have indicated the desirability of providing a target electrode having an essentially spherical curvature so that the surface thereof is always normal to the electron beam; this eliminates a source of serious signal shading previously found in tubes with fiat target electrodes. The provision of such spherically curved target electrode assembly was extremely difficult, if not impossible, when employing sheet mica as the dielectric layer, however, it is relatively easy to coat the inner surface of a spherically curved metal backing plate with a layer of ceramic material. it has, however, been found to be extremely difficult to provide the requisite spherically-curved fine mesh screen for use in the spherically curved target electrode assembly. Even though a properly formed, spherically curved, fine mesh screen is provided, and a metal backing plate having the same spherical curvature is also provided, a difficult feat in and of itself, it is extremely difiicult if not impossible, to provide a ceramic layer of uniform thickness throughout its entire area. Thus, with the employment of a preformed spherically curved fine mesh screen, it is found to be practically impossible to position the screen in intimate contact with 3,152,384 Patented. Get. 13, 1954 the entire surface area of the dielectric layer and in turn to fuze the same thereto.

in addition to the foregoing problems encountered in providing a spherically curved target electrode assembly for a barrier grid storage tube, commercially available glass materials for coating the metal backing plate may not possess a sufficiently high secondary emission, and it therefore may be desirable to coat the surface area of the ceramic layer with a layer of high secondary emissive material. Such materials, however, have a relatively high melting point compared with the softening point of the underlying glass layer, and thus the fine mesh screen cannot be fuzed thereto, as was possible when glass alone was employed as the dielectric layer.

it is therefore an object of our invention to provide an improved method for forming a bowl-shaped fine mesh metal screen for an electron discharge tube.

Another object of our invention is to provide an improved method of making a bowl-shaped target electrode for a barrier grid storage tube in which an initially fiat fine mesh metal screen is directly formed into the bowlhaped metal backing plate and dielectric layer assembly and bonded thereto.

in accordance with the broader aspects of our method, we provide a member having a bowl-shaped cavity therein and a flat fine mesh metal screen is placed over the member so as to cover the cavity. A diaphragm or fiat sheet of relatively thin metal foil is then placed over the screen and a hollow cover member is placed over the foil so as to surround the area of the cavity. The peripheral edge of the cover member is sealed to form a pressure chamber with the foil forming a wall thereof, and gas is introduced under pressure into the cover member over the foil, thereby pressure-forming the foil and the underlying screen into the cavity. In the forming of a tar et electrode assembly for barrier grid storage tubes in accordance with our invention, a bowl-shaped metal backing member is provided with the interior surface thereof coated with a layer of dielectric material. The bowl-shaped metal backing member is then placed in a cavity in the supporting member with the open end thereof facing outwardly and the fiat fine mesh screen, foil and cover member then sequentially placed thereover as above described. Following pressure forming of the screen and foil into the bowl-shaped metal backing plate, the metal backing plate is heated to a sufiicient temperature and for a sufiicient time to bond the screen to at least a portion of the dielectric layer. The heating is then terminated and the assembly cooled to a lower temperature, following which the pressure is removed and the cover member in turn removed. The bowlshaped metal backing plate with the dielectric material layer therein and the screen and foil in turn therein are then removed from the supporting member and the foil is stripped away from the screen.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:

F161 is a cross-sectional view illustrating the apparatus with which the methods of our invention are performed;

FIG. 2 is a fragmentary cross-sectiona1 view of one target electrode assembly produced by a method in accordance with our invention;

FIG. 3 is a fragmentary cross-sectional view, partly broken awa illustrating another method in accordance with our invention;

FIG. 4 is a fragmentary cross-sectional view illustrating a target electrode assembly made in accordance with yet another method of our invention; and

FIG. 5 is a fragmentary cross-sectional view, partly broken away, illustrating still another method of our invention.

Referring now to FIG. 1, the apparatus with which our invention is performed, comprises a base member 12 having a member 14 formed of suitable heat-resistant material, such as lava, positioned thereon. Member 14 has a spherically curved recess or cavity 15 formed in its upper surface with an electrical resistance heating element 18 positioned in suitable grooves 20 communicating therewith. A central opening 22 for accommodating thermo-couple 24 extends through base member 12 and lava member 14 to communicate with cavity 16. Heater 18 is energized by suitable leads 26. One or more ports 28 also extend through base 12 and lava member 14 communicating with the upper surface 3% of lava member 14 outwardly from cavity 16.

The spherically curved metal backing plate 32 with layer 34 of dielectric material coated on its irmer surface is positioned in cavity 15 of lava member 14 with its open end facing upwardly, as shown. In FIG. 1, dielectric layer 34 is shown as being a relatively thin layer of suitable ceramic material, such as relatively low softening point glass.

An initially flat fine mesh metal screen 36 is provided having its peripheral edge mounted on mounting ring 38 which is in turn supported on upper surface of lava member 14 so as to cover the open end of the metal backing plate 32, as shown.

A suitable upstanding ring member 43 is supported on base member 12 surrounding lava member 14, and has a suitable annular sealing gasket 42 seated in a notch in its upper edge, as shown. A sheet of relatively thin metal foil 44, such as aluminum foil, is positioned over fine mesh screen 36, and is supported on annular gasket 42 of ring member 40, as shown. A cover member 46 is provided having an annular wall portion 48 with a suitable annular gasket 50 seated in a groove in its peripheral edge as shown. Cover member 46 is positioned with annular gaskets 42 and 5t sealingly engaging aluminum foil sheet 44, as shown. Cover member 4a is arranged so that aluminum foil sheet 44 is sealingly engaged thereby to forma pressure cavity 52 with the aluminum foil sheet 44 forming one wall thereof, by means of suitable bolts 54 having their lower ends threadingly engaging suitable tapped openings in base 12 and having their upper ends extending through lugs 56 for engagement by suitable thumb screws 58, as shown. Port 60 is provided in the upper wall 62 of cover member 4d for introducing gas under pressure into the pressure chamber 52, thereby to pressure form aluminum foil sheet 44 and the underlying fine mesh metal screen 36 into spherically curved metal backing member 32 and into intimate engagement throughout its surface area with the dielectric layer 34.

Referring now additionally to FIG. 2, in accordance with one method of our invention, metal backing plate 32 may be formed of stainless steel clad copper .050 inch thick and having an outside diameter of 4.532 inches. Dielectric layer 34 is formed of a layer .002 to .005 inch thick of soda-lead-potash glass enamel; we have succesfully used a glass enamel sold by the Corning Glass Company under number 7570 which has a softening point of about 450 C. Screen 36 was a 500 mesh flat copper screen having a thickness of .0005 inch.

The metal backing plate 32 with the glass enamel layer 34 fuzed to its inner surface is positioned in cavity 16, as shown, with screen as and aluminum foil 44, which may have a thickness of .003 inch positioned tliereover, as shown. Cover member 46 is then assembled and tightened sealingly to engage aluminum foil 44 by means of thumb screws 58. In order to prevent oxidation of the line mesh metal screen 36 due to the heat subsequently to be applied, the interior of the metal backing plate 32. with dielectric layer 34 under the aluminum foil 44 is then flushed with a reducing atmosphere. More particularly,

argon, which is a heavy inert gas, bubbled through alcohol is introduced through port 23 for a period of approximately five (5) minutes. The argon is sufliciently heavy to settle in the interior of the metal backing plate 32 with its enamel coating 34. With the argon still being appplied, nitrogen at fifteen (15) pounds per square inch pressure is then applied to the pressure chamber 52 through port 60 for approximately five (5) minutes, which is found sufiicient to pressure-form aluminum foil sheet 44 and the underlying fine mesh screen 36 down into the spherically curved metal backing plate 32 with its enamel coating 34 with the screen being in intimate contact with the enamel coating. With the nitrogen pressure still applied, but reduced in pressure to two (2) pounds per square inch, leads 26 of resistance heater 18 are then energized with a suitable potential to heat the outer surface of metalbacking plate 32 to approximately 450 C., as sensed by thermo-couple 24, over a period of approximately one (1) hour. Once the temperature of the metal backing plate 18 has reached 450 C., the temperature is maintained for approximately five (5) minutes, which has been found to be adequate to fuze the entire surface area of screen 36 to the enamel layer 34. The heating is then terminated with the nitrogen pressure still being applied and the assembly is allowed to cool to about C. through 70 C. over about two (2) hours. T he nitrogen pressure applied to port is then removed, cover member 45 is removed, the metal backing plate 32 with its enamel layer 34 and with screen 36 and aluminum foil 44 formed therein is then removed from lava member 14, and the aluminum foil 44 then stripped or peeled away from the fine mesh screen 36 which is now firmly fuzed to the glass enamel layer 34. The peripheral area of the fine mesh screen 36 extending beyond the peripheral edge of the metal backing plate 32 is then trimmed away to provide the finished target electrode.

Referring now to PEG. 3, it may be desirable to coat the inner surface of the ceramic layer 34 with a relatively thin layer of high secondary emission material, such as zinc sulfide (ZnS) or magnesium fluoride (MgF A zinc sulfide layer approximately one micron thick may be evaporated onto the inner surface of ceramic layer 34 or a magnesium fluoride layer having an approximate thickness of one interference fringe using sodium D light may likewise be evaporated onto the ceramic layer 34. Here, a low softening point soda-lead-potash glass enamel sold by the Corning Glass Company under number 0120 and having a softening point of 630 C. was employed.

In order firmly to bond fine mesh metal screen 36 to the secondary emissive material layer 64, the dielectricside of screen 36 has a thin layer 66 of solder applied thereto, as by evaporating. Here, it will be readily seen that it is desirable that solder layer 66, which preferably is much thinner than the screen, for example less than one micron, should have a melting point lower than the softening point of the underlying ceramic layer 34. We have successfully employed 99% lead as a solder which has a melting point of 327 C., and also an alloy of 77 /2% gold-22 /2% indium, which has a melting point of about 500 C.

Here, metal backing plate 32, with the glass enamel layer 34 and secondary emissive material layer 64, is again positioned in cavity 16 of lava member 14 with screen 36 being positioned thereover, with solder layer 66 facing secondary emissive layer 64, and the remaining apparatus is assembled as described above. The interior of metal backing plate 32 with coatings 34 and 64 therein is again flushed with argon bubbled through alcohol for five minutes, and following the pressureforming of screen 36 and aluminum foil into the metal backing plate 32 with its coatings 34 and 64 and into intimate contact with the secondary emissive material layer 64, the leads 26 of heater 18 are again energized to heat the outer surface of metal backing plate 32 up to the melting point of the solder layer 66; in the case of the lead solder, the backing plate 32 was heated to 350 C. over a period of one hour and this temperature was then held for fifteen minutes. The assembly was then cooled for two hours to close to room temperature and the completed target assembly again removed, it having been found that the fine mesh metal screen 36 was firmly bonded throughout its entire surface area to the secondary emissive material layer 64 by virtue of the solder coating 66 thereon.

Referring now to FIG. 4, the layer 34 of glass enamel is extended to the peripheral edge of the metal backing plate 32, however, the secondary emissive material layer 64 is terminated short of the peripheral edge of the backing plate 32, thereby to expose an annular peripheral area 63 of the ceramic coating 34. Here again, the ceramic coating 34 may be Corning 7570 glass enamel, having a softening point or" approximately 450. Using the apparatus of FIG. 1 and the method described in connection with FIG. 2, the metal backing plate 32 is heated up to a temperature of approximately 450 C. which is considerably lower than the melting point of the secondary enussive material layer 64, but which results in the peripheral edge 7%? of the fine mesh screen 36 being fuzed to the peripheral area 68 of the enamel coating 34.

Since the screen 36 is not bonded or fuzed over its entire surface area to the secondary emissive material layer 64, but rather only fuzed around its peripheral edge to a peripheral area of the enamel coating 34-, it is necessary to insure that the fine mesh screen 36 is forced into intimate engagement with the secondary emissive material layer 64. To accomplish this objective, the metals of the backing plate 32 and of the fine mesh screen 36 are correlated so that the screen has a thermal expan sion coefficient equal to or less than that of the metal backing plate. Thus, the backing plate 32 may be again formed of stainless steel clad copper, and the screen may be formed of nickel, woven mesh stainless steel, or copper. It will be observed that by virtue of the heating element 18 being disposed adjacent the outer surface of the metal backing plate 32, the backing plate will be eated more rapidly and to a somewhat higher temperature than the screen which is separated from the backing plate by the ceramic layer 34 and the secondary emissive material layer 64, and thus the metal of the backing plate will expand during heating a greater amount than will the screen. The peripheral edge of screen '70 is thus fuzed to the peripheral area 68 of the glass enamel layer 34 at a point when the metal backing plate 32 has expanded to its greatest degree. Thus, it will be readily apparent that during cooling, the metal backing plate 32 having expanded more than the screen, will likewise contract to a greater degree than the screen, thus placing the screen 36 in compression and physically forcing the same into intimate engagement with the secondary emissive material layer 64.

Referring now to FIG. 5, rather than terminating secondary emissive material layer 64 short of the peripheral edge of the backing plate 32, it may extend coextensive with the backing plate 32 and the glass enamel layer 34. Here, the glass enamel layer 34 may again be Coming 0120 having a softening point of approximately 559 C. and a belt '72 of lower softening point solder glass, such as Corning 7570 having a softening temperature of about 450 C. may then be arranged embracing the peripheral edge of metal backing plate 32 with its coatings 34 and 54, as shown. The peripheral edge 7% of screen 36 will then be fuzed to the belt 32 in the same manner as the fuzing of peripheral edge 7%} of screen 34 to peripheral area 68 of glass enamel layer 34, as described above in connection with FIG. 4.

It will now be readily apparent that we have provided an apparatus and methods employed thereby for providing a spherically curved target electrode for a barrier grid storage tube in which the fine mesh screen is formed directly into the target electrode so that it intimately engages the surface of the dielectric material layer, and in which the screen is bonded to the assembly, thereby eliminating microphonics previously encountered.

While we have described above the principles of our invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of our invention.

What is claimed is:

1. The method of forming an initially fiat fine mesh metal screen into a bowl-shaped target electrode assembly for a storage tube comprising the steps of: providing a metallic backing plate having a bowl-shaped cavity and a dielectric layer on the surface thereof; placing the backing plate in a support structure; placing a flat fine mesh metal screen over said plate so as to cover said cavity; placing a relatively thin diaphragm over said screen; placing a hollow cover member over said diaphragm so as to surround the area of said cavity and sealing the peripheral edge of said cover member to form a pressure chamber with said diaphragm forming a wall thereof; introducing gas into said cover member over said diaphragm of sufiicient pressure to pressure form said diaphragm and underlying screen into said cavity to bear against said dielectric surface; heating said plate to a sufiicient temperature for a sufficient time to soften said layer and fuse said screen thereto; terminating said heating and cooling said plate to resolidify said layer with said screen fused thereto; removing said pressure and said cover member; and stripping said diaphragm from said screen.

2. The method of claim 1 wherein said diaphragm is formed of metal foil.

3. The method of forming an initially flat fine mesh metal screen into a bowl-shaped target electrode assembly for a storage tube comprising the steps of: placing a bowl-shaped metallic backing plate having a dielectric layer thereon in a cavity in a supporting member with the open end of said metallic plate and layer facing outwardly; placing a fiat fine mesh metal screen over said metallic plate so as to cover said open end thereof; placing a flat sheet of relatively thin metal foil over said screen with its peripheral area engaging said supporting member; placing a hollow cover member over said foil so as to surround the area of said metallic plate; sealing the peripheral edge of said cover member to said supporting member with said peripheral area of said foil sealingly engaged thereb-etween to form a pressure chamber with said foil forming a wall thereof; introducing gas into said cover member over said foil of sufficient pressure to pressure form said foil and underlying screen into contact with said dielectric layer; heating said plate sufliciently to soften said layer to fuse said screen thereto; terminating said heating and cooling said plate to resolidify said layer with said screen fused thereto; removing said pressure and said cover member; removing said metallic plate with said screen and foil therein from said supporting member; and stripping said foil from said screen.

4. The method of formin an initially flat fine mesh screen into a bowl-shaped metal backing plate member for a storage tube target electrode assembly having its inner surface coated with a layer of dielectric material, and of bonding the thus formed screen to at least a portion of said member comprising the steps of: placing the bowl-shaped member in a cavity in a supporting metal backing plate member having said dielectric inner surface layer with the open inner surface of said bowl-shaped member facing outwardly; placing a fiat fine mesh metal screen on said supporting member over said bowl-shaped member so as to cover said open inner surface thereof; placing a flat sheet of relatively thin metal foil over said screen with its peripheral area supported by a portion of said supporting member; placing a hollow cover mem ber over said foil so as to surround the area of said bowlshaped member; sealing the said cover member to said amassisupporting member to form a pressure chamber with said foil forming a wall thereof; introducing gas into said cover member over said foil of suflicient pressure to pressure form said foil and underlying screen into said bowlshaped member and into intimate engagement with said dielectric material layer; heating said bowl-shaped member to a suflicient temperature and for a time sufficient to soften said dielectric layer and bond said screen to at least a portion of said dielectric layer; terminating said heating and cooling said bowl-shaped member to a lower temperature to resolidify said layer with said screen bonded thereto; removing said pressure and cover member; removing said bowl-shaped member with said bonded screen and said foil therein from said supporting member; and stripping said foil from said screen.

5. The method of claim 4 further comprising the step of introducing a reducing atmosphere into said bowlshaped member under said screen and foil prior to said pressure-forming thereof.

6. The method of claim 5 wherein said gas is nitrogen and said reducing atmosphere is argon bubbled through alcohol.

7. The method of claim 4 wherein said dielectric material layer is a ceramic material, wherein said heating of said bowl-shaped member is to a sufiicient temperature and for a time sufficient to soften said layer thereby to fuse the entire area of said screen within said bowl-shaped member to said layer, and wherein said cooling is to a temperature sufliciently low to resolidify said layer with said screen being fused thereto.

7 8. The method of claim 4 wherein said dielectric material layer comprises a coating of ceramic material on the inner surface of said bowl-shaped member and a coating of high secondary emissive material on said ceramic coating, wherein said fiat screen has a thin layer of solder applied on its surface facing said bowl-shaped member, and wherein said heating is to a sufiieient temperature and for a time sufficient to melt said solder layer but below the softening temperature of said ceramic coating whereby the entire area of said screen is soldered to said secondary emissive coating.

9. The method of claim 4 wherein said dielectric material layer comprises a coating of relatively low soften ing point ceramic material covering the entire interior surface of said bowl-shaped member and extending substantially to the peripheral edge thereof and a coating of relatively high melting point high secondary emissive material on said ceramic coating but terminating short of the peripheral edge of said bowl-shaped member thereby exposing a peripheral belt of said ceramic material, Wherein said heating of said bowl-shaped member is to a sufficient temperature and for a time sufficient to soften said ceramic coating but not to melt said secondary emissive material coating thereby fusing said screen to said peripheral belt of said ceramic material, and wherein said cooling is to a temperature sufficiently low to resolidify said ceramic material with said screen being fused to said peripheral belt thereof.

10. The method of claim 4 wherein said dielectric material layer comprises a coating of ceramic material covering the entire interior surface area of said bowl-shaped member and a coating of high secondary emissive material covering the entire surface area of said ceramic coating, wherein the peripheral edge of said bowl-shaped member has a belt of ceramic material embracing the same and having a softening point lower than that of said ceramic and secondary emissive coatings, wherein said heating of said bowl-shaped member is to a sufiicient temperature and for a time sufiicient to soften said belt of ceramic material but not to soften said ceramic and secondary emissive coatings thereby fusing said screen to said belt of ceramic material, and wherein said cooling is to a temperature sulficiently low to resolidify said ceramic belt with said screen being fused thereto.

11. The method of claim 4 wherein a peripheral belt of saidlayer adjacent the peripheral edge of said bowlshaped member is formed of relatively low softening point ceramic material, wherein said heating is to a suihcient temperature and for a time suficient to soften said belt of ceramic material thereby fusing said screen thereto, wherein said heating is applied closely adjacent the outer surface of said bowl-shaped member and the metal of said screen and bowl-shaped member is correlated so that said bowl..haped member expands during heating a greater amount than said screen, and wherein said cooling is to a temperature sufficiently low to resolidify said ceramic belt with said screen being fused thereto, and to cause the metal of said bowl-shaped member to contract more than said screen thereby to place said screen in compression to force the same into intimate engagement with the entire surface area of said dielectric layer.

12. The method of making a bowl-shaped target electrode for a barrier grid storage tube comprising the steps of: providing a bowl-shaped backing plate member formed of relatively thin metal having its inner surface coated with a relatively thin layer of relatively low softening point ceramic material; placing the bowl-shaped member in a cavity in a supporting member with the open end of said bowl-shaped member facing outwardly, placing a flat fine mesh metal screen over said bowl-shaped member so as to cover said open end thereof with the peripheral area of said screen supported by the area of said supporting member surrounding said cavity; placing a flat sheet of relatively thin metal foil over said screen with the peripheral area of said foil supported by said supporting member outwardly of said screen; placing a hollow cover member over said foil so as to surround the area of said bowl-shaped member; sealing the peripheral edge of said cover member to said supporting member with said peripheral area of said foil engaged therebetween to form a pressure chamber with said foil forming a wall thereof; introducing a heavy reducing gas to the interior of said bowl-shaped member under said foil and flushing the same for a predetermined time; introducing gas under sufiicient pressure into said cover member over said foil for a time suiiicient to pressure-form said foil and underlying screen into intimate engagement with said ceramic layer; heating the outer surface of said bowl-shaped member to a sufiicient temperature and for a time sufiicient to soften said ceramic layer th reby to fuse the entire area of said screen within said bowl-shaped member to said ceramic layer; terminating said heating and cooling said bowlshaped member to a temperature sufficiently low to resolidify said ceramic layer with said screen fused thereto; removing said pressure and said cover member; removing said bowl-shaped member with said screen and foil therein from said supporting member; stripping said foil from said screen, and trimming the peripheral area of said screen projecting from the peripheral edge of said bowlshaped member.

13. The method of making a bowl-shaped target electrode for a barrier grid storage tube comprising the steps of: providing a bowl-shaped member formed of relatively thin metal having its inner surface coated with a relatively thin layer of relatively low softening point ceramic material; coating said ceramic layer with a relatively thin layer of high secondary emissive material; placing the bowlshaped member in a cavity in a supporting member with the open end of said bowl-shaped member facing outwardly; providing a flat fine mesh metal screen; coating one surface of said screen with a relatively thin layer of solder having a melting point lower than the softening point of said ceramic layer; placing said screen over said bowl-shaped member so as to cover said open end thereof with said solder layer facing the same and with the peripheral area of said screen supported by the area of said supporting member surrounding said cavity; placing a flat sheet of relatively thin metal foil over said screen with the peripheral area of said foil supported by said supporting member outwardly of said screen; placing a hollow cover member over said foil so as to surround the area of said bowl-shaped member; sealing the peripheral edge of said cover member to said supporting member with said peripheral area of said foil engaged therebetween to form a pressure chamber with said foil forming a wall thereof; introducing a heavy reducing gas to the interior of said bowl-shaped member under said foil and flushing the same for a predetermined time; introducing gas under suflicient pressure into said cover member over said foil for a time sufficient to pressure-form said foil and underlying screen into intimate engagement with said secondary emissive layer; heating the outer surface of said bowl-shaped member to a sufiicient temperature and for a time sufficient to melt said solder layer but below the softening temperature of said ceramic layer thereby to solder the entire area of said screen within said bowlshaped member to said secondary emissive layer; terminating said heating and cooling said bowl-shaped memher to a lower temperature at which said screen is bonded to said secondary emissive layer; removing said pressure and said cover member; removing said bowl-shaped member with said screen and foil therein from said supporting member; tripping said foil from said screen; and trimming the peripheral area of said screen projecting from the peripheral edge of said bowl-shaped member.

14. The method of making a bowl-shaped target electrode for a barrier grid storage tube comprising steps of: providing a bowl-shaped backing plate member formed of relatively thin metal having its inner surface coated with a relatively thin layer relatively low softening point ceramic material; coating said ceramic layer with a relatively thin layer of high secondary emissive material which terminates short of the outer peripheral edge of said bowl-shaped member thereby exposing a peripheral area of said ceramic layer; placing the bowl-shaped member in a cavity in a supporting member with the open end of said bowl-shaped member facing outwardly; placing a flat fine mesh metal screen over said bowl-shaped member so as to cover said open end thereof with the peripheral area of said screen supported by the area of said supporting member surrounding said cavity; placing a fiat sheet of relatively thin metal foil over said screen with the peripheral area of said foil supported by said supporting member outwardly of said screen; placing a hollow cover member over said foil so as to surround the area of said bowl-shaped member; sealing the peripheral edge of said cover member to said supporting member with said peripheral area of said foil engaged therebetween to form a pressure chamber with said foil forming a wall thereof; introducing a heavy reducing gas to the interior of said bowl-shaped member under said foil and flushing the same for a predetermined time; introducing gas under sufficient pressure into said cover member over said foil for a time sufiicient to pressure-form said foil and underlying screen into intimate engagement with said secondary emissive layer and said peripheral area of said ceramic layer; heating the outer surface of said bowl-shaped member to a suflicient temperature and for a time sufiicient to fuse said screen to said peripheral area of said ceramic layer, the metals of said screen and said bowl-shaped member being correlated so that said bowl expands more than said screen during heating; terminating said heating and cooling said bowl-shaped member to a temperature sufiiciently low to resolidify said peripheral area of said ceramic layer with said screen fused thereto, and to cause the metal of said bowl-shaped member to contract more than said screen thereby to place said screen in compression to force the same into intimate engagement with the entire surface area of said secondary emissive layer; removing said pressure and said cover member; removing said bowl-shaped member with said screen and foil therein from said supporting member; stripping said foil from said screen; and trimming the peripheral area of said screen projecting from the peripheral edge of said bowl-shaped member.

15. The method of making a bowl-shaped target electrode for a barrier grid storage tube comprising the steps of: providing a bowl-shaped backing plate member formed of relatively thin metal having its inner surface coated with a relatively thin layer of relatively low softening point ceramic material; coating said ceramic layer with a relatively thin layer of high secondary emissive mate rial; applying a peripheral belt of ceramic material over said secondary emissive material adjacent the peripheral edge of said bowl-shaped member which has a softening point lower than said ceramic layer; placing the bowlshaped member in a cavity in a supporting member with the open end of said bowl-shaped member facing outwardly; placing a hat fine mesh metal screen over said bowl-shaped member so as to cover said open end thereof with the peripheral area of said screen supported by the area of said supporting member surrounding said cavity; placing a fiat sheet of relatively thin metal foil over said screen with the peripheral area of said foil supported by said supporting member outwardly of said screen; placing a hollow cover member over said foil so as to surround the area of said bowl-shaped member; sealing the peripheral edge of said cover member to said supporting member with said peripheral area of said foil engaged therebetween to form a pressure chamber with said foil forming a wall thereof; introducing a heavy reducing gas to the interior of said bowl-shaped member under said foil and flushing the same for a predetermined time; in troducing gas under sufficient pressure into said cover member over said foil for a time sufficient to pressureform said foil and underlying screen into intimate engagement with said secondary emissive layer and said peripheral belt; heating the outer surface of said bowlshaped member to a sufficient temperature and for a time suflicient to fuse said screen to said peripheral belt but below the softening temperature of said ceramic layer, the metals of said screen and said bowl-shaped member being correlated so that said bowl expands more than said screen during heating; terminating said heating and cooling said bowl-shaped member to a temperature sufliciently low to resolidify said peripheral belt with said screen fused thereto, and to cause the metal of said bowl-shaped member to contract more than said screen thereby to place said screen in compression to force the same into intimate engagement with the entire surface area of said secondary emissive layer; removing said pressure and said cover member; removing said bowl-shaped member with said screen and foil therein from said supporting member; stripping said foil from said screen; and trimming the peripheral area of said screen projecting from the peripheral edge of said bowl-shaped member.

References Cited in the file of this patent UNITED STATES PATENTS 895,412 Badger Aug. 11, 1908 2,975,259 Osborn Mar. 14, 1961 3,017,687 Day Jan. 23, 1962 

1. THE METHOD OF FORMING AN INITIALLY FLAT FINE MESH METAL SCREEN INTO A BOWL-SHAPED TARGET ELECTRODE ASSEMBLY FOR A STORAGE TUBE COMPRISING THE STEPS OF: PROVIDING A METALLIC BACKING PLATE HAVING A BOWL-SHAPED CAVITY AND A DIELECTRIC LAYER ON THE SURFACE THEREOF; PLACING THE BACKING PLATE IN A SUPPORT STRUCTURE; PLACING A FLAT FINE MESH METAL SCREEN OVER SAID PLATE SO AS TO COVER SAID CAVITY; PLACING A RELATIVELY THIN DIAPHRAGM OVER SAID SCREEN; PLACING A HOLLOW COVER MEMBER OVER SAID DIAPHRAGM SO AS TO SURROUND THE AREA OF SAID CAVITY AND SEALING THE PERIPHERAL EDGE OF SAID COVER MEMBER TO FORM A PRESSURE CHAMBER WITH SAID DIAPHRAGM FORMING A WALL THEREOF; INTRODUCING GAS INTO SAID COVER MEMBER OVER SAID DIAPHRAGM OF SUFFICIENT PRESSURE TO PRESSURE FORM SAID DIAPHRAGM AND UNDERLYING SCREEN INTO SAID CAVITY TO BEAR AGAINST SAID DIELECTRIC SURFACE; HEATING SAID PLATE TO A SUFFICIENT TEMPERATURE FOR A SUFFICIENT TIME TO SOFTEN SAID LAYER AND FUSE SAID SCREEN THERETO; TERMINATING SAID HEATING AND COOLING SAID PLATE TO RESOLIDIFY SAID LAYER WITH SAID SCREEN FUSED THERETO; REMOVING SAID PRESSURE AND SAID COVER MEMBER; AND STRIPPING SAID DIAPHRAGM FROM SAID SCREEN. 